The present invention concerns first a method of injection-molding three-layer moldings, especially bottle blanks. They have inner and outer layers of a "material A" and a middle layer of a "material B". Material A can be polyethylene terephthalate (PET) for example. Material B is a material, a copolymer of ethylene and polyvinyl alcohol (EVAL) for example, that acts as a barrier against the gases O.sub.2, CO.sub.2, and SO.sub.2, and against water vapor (H.sub.2 O). The method employs a device that comprises at least one mold and at least one flat hot runner. The mold has several cavities and the hot runner has the same number of dies. The two materials are supplied to the cavities through separate hot-runner systems. The invention also concerns such a device for carrying out the method.
The present invention is not, however, restricted either to bottle blanks or to PET. It can be used for injection molding moldings of any other plastic or combination of plastics for any other purpose. The invention can for example be employed for molding multiple-layer moldings of PET and polyethylene naphthalate (PEN). PEN is, like PET, polyester, although it has much better heat-resisting properties, overall mechanical properties, and barrier-forming properties. Although still fairly new, PEN has already been approved by the various governmental authorities as a packaging material in the foodstuffs industry. Due to its outstanding properties, even relatively small portions of PEN by weight of the blank contribute considerably to the quality of the finished bottle. Bottles made of PET or PEN are particular appropriate for bottling fruit juices hot and for storage in hot climates.
The middle layer in such blanks constitutes a core, and the inner and outer layer integral skins. The inner layer can also constitute a barrier layer. Such layers, especially those that create a barrier against the gases O2, CO2, and SO2, and against water vapor (H2O), are very important to blanks of this type because they decrease permeability.
The inner layers employed in known methods of the aforesaid genus are relatively thick. The thickness makes the blanks considerably more expensive because materials that can act as barriers are substantially more expensive than those that can be employed for outer layers. Another drawback of the known methods is the impossibility of distributing the materials employed for the barrier layers at all uniformly throughout all the cavities in a multiple-cavity mold (a mold with 32 or 48 cavities).
Since the material B to be employed as a barrier layer need not be exposed to as much heat as the material A in the outer layer, the two layers are supplied to the cavities through separate hot-runner systems. This causes considerable problems. One device (known from EO 0 246 512 B1) for simultaneously injection molding blanks with several layers accordingly features a separate source and separate ducts. A lot of material B, however, is always left in the die and in the immediately adjacent ducts after each cycle, and the material is exposed to too much heat before being introduced into the cavities.